Ok, first of all mail header enricher is used to send mail not to receive.
For your use case (routing based on the subject) there are quite a few simple ways. Here is a couple.
1. Header Enricher -> Header Value Router
Since the payload of the Message is a MimeMessage you can simply execute payload.getSubject() using SpEL and set it as SUBJECT header. Then use HeaderValueRouter and route based on the value of SUBJECT header.
<int:header-enricher input-channel="mailReceiveChannel" output-channel="routingChannel">
<int:header name="SUBJECT" expression="payload.getSubject()"/>
<int:header-value-router input-channel="mailReceiveChannel" header-name="SUBJECT">
<int:mapping channel="channelA" value="TICKET-100"/>
<int:mapping channel="channelB" value="TICKET-200"/>
2. RecipientListRouter with selector expression
If your selection/routing logic is simple enough to be handled by SpEL, then as you can see a simple selector-expression would eliminate the need for enriching the headers and instead use recipient-list-router with boolean SpEL expression thus still routing to a single channel.
<int:recipient channel="ticket100-channel" selector-expression="payload.getSubject().endsWith('100')"/>
<int:recipient channel="ticket100-channel" selector-expression="payload.getSubject().endsWith('200')"/>